POLLUTION and contamination with mercury is an interesting and complex story, for in a toxicological sense there are "two mercurys." On the one hand, mercuric chloride and methylmercury are highly poisonous. Compounds of mercury, especially in fish and shellfish, have caused widely publicized incidents of poisoning in human beings. On the other hand, mercurous chloride was used as a laxative, and mercury and its compounds are employed in chemotherapy, in diuretics, and by dentists.
All of the poisonings with mercury were tragic; some were caused by flagrant and inexcusable neglect by industry; others by illiteracy or even stupidity. The uproar over mercury in swordfish and canned tuna was unnecessary and somewhat ludicrous. The discovery of the protective effect of selenium was fascinating, and did much to clear the air.
"All the rivers run into the sea, yet the sea is not full; unto the place from whence the rivers come,
Jukes TH. Mercury in Fish. JAMA. 1975;233(9):1001–1002. doi:10.1001/jama.1975.03260090067031